An ultrasonic transducer transmits and receives an ultrasonic wave and is used in a diagnosis device for a tumor in a human body, for example.
Conventionally, an ultrasonic transducer using vibration of a piezoelectric body is used. Because of advance of a MEMS technique in recent years, a capacitive micromachined ultrasonic transducer (CMUT) in which a vibrating part having a configuration interposing a void between two layer electrodes in upper and lower portions is fabricated over a silicon substrate is actively developed toward practical use.
In comparison with a conventional transducer using a piezoelectric body, the CMUT has advantages such as a wide usable ultrasonic frequency band and a high resolution. And, since the CMUT is fabricated using an LSI process technique, microfabrication is possible. In particular, in a case where ultrasonic elements are arranged in an array and each element is controlled independently, the CMUT is considered to be indispensable. This is because although wirings for respective elements are required and the number of the wirings in the array becomes huge, wirings and even a processing circuit for a signal from an ultrasonic transmission-reception unit can be embedded in one chip in the CMUT.
A technique related to such an ultrasonic transducer is disclosed, for example, in a specification of U.S. Pat. No. 6,271,620B1 (Patent Document 1).
And, a sensor in which an insulator film and the void are interposed between upper and lower electrodes is disclosed in Japanese Patent Application Laid-Open Publication No. 2003-28740 (Patent Document 2) and Japanese Patent Application Laid-Open Publication No. 2004-361115 (Patent Document 3).